Sudden voltage drops can be caused on the connection to the electrical output network of a park of electric generators such as wind turbines, as a result of short circuits on the network. In such cases, implementing the proper protection disconnects the defective part of the network, producing a new voltage step in the opposite direction and restoring the voltage to the regular level, prior to the defect.
The combination of both steps is known as a “voltage sag” and can be defined by two parameters: both the magnitude and the duration of the voltage step.
A “voltage sag” can cause a wind farm to be disconnected from the network bringing the consequent harmful effects along with it, this therefore requires the correct behaviour of wind turbines in response to voltage sags to be tested.
Discussed below are various known devices for generating voltage sags used for different purposes.
Patent application WO0060430 describes a system for verifying the sensitivity of the components of a glass production plant in response to voltage sags that may occur, comprising a double cascade of industrial autotransformers connected to each phase of the current with at least two bipolar switches, which generates voltage sags on the machine output in proportion to the values of the individual autotransformers located on the cascades. The system provides the option to vary the amplitude and duration of the sag, along with the option to cause different voltage sags for each phase.
Document U.S. Pat. No. 5,886,429 proposes a computer controlled and monitored testing station for testing the response of electronic equipment, connected to said station, to voltage sags and swells generated therein.
Document U.S. Pat. No. 5,920,132 describes a device which is able to generate a reduced voltage by means of an autotransformer on low power industrial equipment, only valid for low voltage.
None of the aforementioned devices can be applied for verifying the behaviour of electrical generator machines such as wind turbines in response to different types of voltage sags. One must consider that in each country there are different standards on this matter, therefore, a wind turbine manufacturer must be able to verify the effects of different types of voltage sags on its machines in order to guarantee compliance with the different standards. This invention proposes a solution to this problem.